In public finance, a lender of last resort (LOLR) is a financial entity, generally a central bank, that acts as the provider of liquidity to a financial institution which finds itself unable to obtain sufficient liquidity in the interbank lending market when other facilities or such sources have been exhausted. It is, in effect, a government guarantee to provide liquidity to financial institutions. Since the beginning of the 20th century, most central banks have been providers of lender of last resort facilities, and their functions usually also include ensuring liquidity in the international markets in general.

The objective is to prevent economic disruption as a result of financial panics and bank runs spreading from one bank to the others due to a lack of liquidity in the first one.

There are varying definitions of a lender of last resort, but a comprehensive one is that it is "the discretionary provision of liquidity to a financial institution (or the market as a whole) by the central bank in reaction to an adverse shock which causes an abnormal increase in demand for liquidity which cannot be met from an alternative source".

While the concept itself had been used previously, the term "lender of last resort" was supposedly first used in its current context by Sir Francis Baring, in his Observations on the Establishment of the Bank of England, which was published in 1797.

Although Alexander Hamilton, in 1792, was the first policymaker to explain and implement a lender of last resort policy, the classical theory of the lender of last resort was mostly developed by two Englishmen in the 19th century: Henry Thornton and Walter Bagehot. Although some of the details remain controversial, their general theory is still widely acknowledged in modern research and provides a suitable benchmark. Thornton and Bagehot were mostly concerned with the reduction of the money supply. That was because they feared that the deflationary tendency caused by a reduction of the money supply could reduce the level of economic activity. If prices did not adjust quickly, it would lead to unemployment and a reduction in output. By keeping the money supply constant, the purchasing power remains stable during shocks. When there is a shock-induced panic, two things happen:

Thornton first published An Enquiry into the Nature and Effects of the Paper Credit of Great Britain in 1802. His starting point was that only a central bank could perform the task of lender of last resort because it holds a monopoly in issuing bank notes. Unlike any other bank, the central bank has a responsibility towards the public to keep the money supply constant, thereby preventing negative externalities of monetary instability, such as unemployment, price instability, bank runs, and financial panic.

Bagehot was the second important contributor to the classical theory. In his book Lombard Street (1873), he mostly agreed with Thornton without ever mentioning him but also develops some new points and emphases. Bagehot advocates: "Very large loans at very high rates are the best remedy for the worst malady of the money market when a foreign drain is added to a domestic drain." His main points can be summarized by his famous rule: lend "it most freely... to merchants, to minor bankers, to 'this and that man', whenever the security is good".

Thomas M. Humphrey, who has done extensive research on Thornton's and Bagehot's works, summarizes their main proposals as follows: (1) protect the money supply instead of saving individual institutions; (2) rescue solvent institutions only; (3) let insolvent institutions default; (4) charge penalty rates; (5) require good collateral; and (6) announce the conditions before a crisis so that the market knows exactly what to expect.